This invention relates to high-density light emitting diode (LED) arrays and, more particularly, to an LED array that has improved collection and collimation of light.
High-density UV LED arrays may be used for a variety of applications, including, e.g., curing applications ranging from ink printing to the fabrication of DVDs and lithography. Many such applications require a high optical power density at the working surface. However, such power density tends to be unavailable from a typical LED as such LED alone generally is characterized by light distribution that is undesirably divergent.
For example, FIG. 1 is a graphic illustration showing radiation distribution for two typical LEDs 22, 24 mounted on a surface 26 without refractive or reflective optics. Ideally, particularly for applications as above described, the light from the LEDs 22, 24 would be distributed substantially 90 degrees from substrate 26. However, the LEDs 22, 24 will typically emit highly divergent light. In FIG. 1, this is illustrated by curves 28 and 30. Curve 28 is a representative example of radiation distribution from first LED 22 and curve 30 is a representative example of radiation distribution from second LED 24. Because much of radiation from LEDs 22, 24 is emitted at highly oblique angles, the optical power density falls off relatively quickly as a function of the distance of the work surface from the LED. This fall off is graphically illustrated in FIG. 2 for an LED array without refractive or reflective optics.
Typically, the performance illustrated by FIGS. 1 and 2 may vary as the system is changed. For example, the performance would tend to vary if different LEDs are used and if the LEDs are placed in different arrays (e.g., depending on the number and spacing of LEDs in the array). In any case, particularly for the aforementioned applications, divergence and fall off similar to that illustrated by FIGS. 1 and 2 will tend to be undesirable.
To achieve the optical power density typically required in the aforementioned applications, an LED array exhibiting such divergence could and often is located physically close to the work surface. That is, the proximity of the array to the work surface would be closer than if the array did not exhibit such divergence. Moreover, such close proximity generally is undesirable, including because it will typically necessitate mechanical changes to tooling and/or shielding to accommodate such proximity. However, locating the LED array too far from the work surface may diminish the optical power density to undesired levels, which levels may hinder or preclude proper operation in the application.
There are known methods of achieving higher optical power density. For example, some LEDs are used with Lambertian optical outputs to achieve a higher optical power density. However, such devices are less efficient in electrical to optical conversions as well as being less thermally efficient. Another method of achieving higher optical power density is shown if FIG. 3 in which an array of refractive optical elements 32 is located above an array of LEDs 34 in which each LED 34 is associated with an optical element 32. Each optical element 32 collects and collimates the light from its associated LED 34. However, this method is inefficient because light from LEDs is highly divergent causing much of the light to fall outside the numerical aperture of the lenses. The numerical aperture of a lens is defined as the sine of the angle between the marginal ray (the ray that exits the lens system at its outer edge) and the optical axis multiplied by the index of refraction (n) of the material in which the lens focuses. In order to more effectively collect and collimate the light the optical component must have a very high numerical aperture resulting in a lens that has a very large diameter and a very short focal length. In practice, it is not possible to manufacture a refractive optical element that collects all of the light from an LED because that would require the angle between the optical axis and the marginal ray to be 90 degrees, implying a lens of either a zero focal length or an infinite diameter.
Another common approach to collecting and collimating light from an LED is to use a parabolic reflector as shown in FIG. 4. AN LED 36 is mounted in parabolic reflector 38 so that light rays 40 emitted from LED 36 are collected and collimated. Unlike refractive optics, reflective optics generally collect all the light from the LED, even at very highly oblique angles. However, known reflective optics are not used in a tightly packed or dense array because of their size. For example, a typical application of such reflective optics is with LED-mounted flashlights in which the reflective optic collimates light from only a single LED.
Additionally, in known optical devices the reflector is separate from the electrical circuitry of the device. For example, such devices typically utilize a macro-reflector for an entire array of LEDs. The optical efficiency of these devices is lowered because each LED does not have an associated reflector. Additionally, the volume of space required for the macro-reflector is very large which increases the cost of manufacturing.